This invention relates to an optical glass and, more particularly, to an optical glass which has optical constants of a refractive index (nd) within a range from 1.60 to 1.69 and an Abbe number (νd) within a range from 35 to 45, has a low glass transition point (Tg) and is suitable for mold press forming.
In recent remarkable development of more compact and light-weight optical instruments, more aspherical lenses tend to be used for the purpose of reducing the number of lenses constituting an optical system of an optical instrument. For manufacturing an aspherical lens, the main current of the industry is to heat and thereby soften a preform obtained from a glass gob or glass block and press the softened preform with a mold having a high precision surface for transferring its mold surface to the preform. An aspherical lens obtained by this method can be processed to a product of a desired shape at a high productivity without grinding and polishing, or with minimum grinding and polishing and, therefore, a large scale production of a product at a low cost can be expected.
There are generally two methods for manufacturing the preform. One is a dropping method according to which, as described, e.g., in Japanese Patent Application Laid-open Publication No. Hei 6-122526, melted glass is dropped from a tip of a flow tube, received and molded by a mold and then is cooled to provide a glass preform. According to this method, a preform is directly obtained while the glass is hot and, therefore, a preform can be manufactured at a high productivity in a large scale production. Besides, since a glass preform thus obtained has a lens shape which is either spherical or convex in both surfaces, an amount of change in the shape during the precision mold press forming can be held at the minimum.
The other method is to obtain a preform by cutting a glass block. This method has the problem that it requires increased steps of processing from cutting of a glass block to a step of processing to a shape which is dose to a final lens shape but, since processing to a lens shape dose to a final lens shape is conducted, an amount of change in the shape can be reduced in forming various shapes of lenses besides a lens shape which is convex in both surfaces.
In producing a glass product by precision mold pressing, it is necessary to heat and press a glass preform under a high temperature for transferring a high precision mold surface to the glass product. For this reason, a mold used for this purpose is subjected to high temperature and pressure and, as a result, the surface of the mold tends to be oxidized and corroded in softening the preform by heating. This makes it difficult to maintain a high precision surface of the mold and, therefore, the number of times of replacing the mold increases and, therefore, a large scale production of the glass product at a low cost becomes difficult. When a glass transition point (Tg) of glass which constitutes a glass preform is high, there arises the problem that damage occurs to a mold and also to a mold releasing film provided on the inner surface of the mold due to the high temperature environment in mold pressing.
Glasses having various optical constants are sought as glass used for aspherical lenses. Among them, there is a strong demand for glass having optical constants of a refractive index (nd) within a range from 1.60 to 1.69 and an Abbe number (νd) within a range from 35 to 45. Known in the art of glasses which have such optical constants are a SiO2—B2O3—(TiO2+ZrO2)—SrO—BaO—R′2O glass composition (Japanese Patent Application Laid-open Publication No. Hei 5-17176) and a SiO2—B2O3—ZrO2—Nb2O5 glass composition (Japanese Patent Application Laid-open Publication No. Hei 10-130033). The glass transition points (Tg) of examples disclosed in these prior art glasses are very high ones exceeding 500° C. and these glasses are not suitable for mold pressing. On the other hand, Japanese Patent Application Laid-open Publication No. Hei 10-265238 discloses an optical glass having a relatively low Tg. This glass, however, is not suitable for mold pressing because of poor resistance to devitrification.
A glass containing PbO tends to be fused to the mold during mold pressing, and therefore, it is difficult to use the mold repeatedly and hence it is not suitable as an optical glass for mold pressing. In a glass containing F2, the F2 ingredient evaporates selectively from the surface of glass melt and thereby causes cloudiness on the surface of a preform when the preform is produced from the glass melt, or, when the preform is molded by mold pressing, the F2 ingredient evaporates and is deposited on the surface of the mold to cause cloudiness on the surface of the mold. For these reasons, the glass containing F2 is not suitable as an optical glass for mold pressing.
It is, therefore, an object of the present invention to provide an optical glass which has eliminated the above described disadvantages of the prior art optical glasses, has optical constants of a refractive index (nd) within a range from 1.60 to 1.69 and an Abbe number (νd) within a range from 35 to 45, has a low transition point (Tg), has excellent resistance to devitrification, and is suitable for mold pressing.